Regular vacuum cleaners remove loose dirt, but they hardly touch the dirt and oils that stick to carpet fibers. Carpet cleaners are supposed to go a step further: They typically spray on a detergent, sometimes working it in with moving brushes then vacuum up the dirty solution.
A machine's effectiveness depends in part on how aggressively it scrubs to loosen dirt and how powerfully its suction pulls dirt in. But too much aggressiveness could damage carpeting. (Powerful suction doesn't pose such a problem.) Some of the machines we've tested don't get at dirt at the bottom of a carpet, but instead remove mainly what you see near the surface.
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